nope not encountered b4... assume you have verified files via steam... if so then...a couple of Q's first.. is it a programmable mouse? Also I assume no mods? If windows, go into settings, mouse, advanced and switch the buttons around (just to test) does problem now appear on other mouse button now.. (note you can see dubious mouse clicks in test window..)or does it magically dissapear...(watch out for Click Lock too while u there i personally never us click lock) ..if it all looks good in settings play with buttons switched and see if it continues..

No, no mods installed ever, and the mouse isn't programmable. I'll try your other suggestions though and see what happens. Thanks for the reply.